Polychlorinated biphenyls (PCBs) are a group of synthetic compounds formerly used for almost half a century in a wide variety of products, including hydraulic fluids, plasticizers, adhesives, fire retardants, wax extenders, dedusting agents, pesticide extenders, inks, lubricants and cutting oils. Their high chemical and thermal stability at first contributed to their commercial usefulness as industrial chemicals, but later, after possible toxicity was demonstrated, to their persistence in the environment and potential health effects. The compounds are hydrophobic and have accumulated primarily in soils and sediments where they bind with organic matter.
Polychlorinated biphenyls are composed of biphenyl molecules containing from 1 to 10 chlorines. The vast majority of PCBs in the environment are derived from once commercial mixtures that contain numerous isomers and homologs generally referred to as congeners. The positions at which chlorine may be attached to the biphenyl nucleus are shown below: ##STR1##
Detoxification of PCBs begins with dechlorination, which involves the stepwise removal of chlorines from the biphenyl nucleus wherein chlorine atoms are replaced with hydrogen atoms. Biological dechlorination of PCBs in contaminated sites is now well documented, and has been observed in some environments (Brown, J. F., et al., Env. Tox. Chem. 6:579-593 (1987), and Brown, J. F., et al., Science 236: 709-712 (1987). However, most naturally occurring microbially mediated dechlorination processes exhibit limited specificity and are generally restricted to removal of para- or meta-chlorines located adjacent to other chlorines, hence residual meta- and/or para-chlorines remain.
To increase dechlorination, stimulation of microbial dechlorination using various compounds such as brominated, or iodinated biphenyls and/or iodo- or bromobenzoic acids has been suggested (U.S. Pat. No. 5,227,069, and U.S. Pat. No. 5,484,729. Though beneficial in speeding up biological dechlorination, the dechlorination processes stimulated are typically limited to removal of meta- or para-chlorines located adjacent to other chlorines, and the like, particularly removal of meta-chlorines. Extensive and desirable removal of all meta- and para-chlorines does not occur, and the end-products contain 3-, 25-, and 235-chlorophenyl groups or 4-, 24-, and 246-chlorophenyl groups.
It would be beneficial to stimulate the microbial dechlorination of PCB congeners that have residual meta or para-chlorines, particularly to stimulate microbial para-dechlorination in concert with other types of microbial dechlorination involving other positions on the biphenyl ring, and "lonely para" dechlorination, i.e., para-dechlorination of rings that have no meta-chlorines.